1. Field of the Invention
The present invention relates to a mobile communication system and a communication method performed in the mobile communication system in which a radiation pattern of a signal output from an antenna of a base station and a radiation pattern of a signal output from an antenna of a mobile station such as a portable telephone or a portable information terminal are controlled in a communication between the base station and the mobile station to improve a communication quality in both the base station and the mobile station.
2. Description of Related Art
FIG. 7 is an explanatory view showing a signal transmission from a mobile station to a base station in a conventional mobile communication system, for example, disclosed in Published Unexamined Japanese Patent Application H9-219615 (1997), and FIG. 8 is an explanatory view showing the reception of a signal output from a base station in a mobile station in the conventional mobile communication system. In FIG. 7 and FIG. 8, 1 indicates an antenna of a base station. A directivity control of an adaptive antenna is performed for the antenna 1 of the base station in signal transmission and reception operations of the base station. 2 indicates an antenna of a mobile station such as a portable telephone or a portable information terminal. 3 indicates an interference wave source for reflecting a transmission wave output from the antenna 1 of the base station and a transmission wave output from the antenna 2 of the mobile station. R1 denotes a space distribution indicating a radiation characteristic of the antenna 1 of the base station for a signal received in the antenna 1 of the base station. T2 denotes a space distribution indicating a radiation characteristic of the antenna 1 of the base station for a signal transmitted from the antenna 1 of the base station. T1 denotes a space distribution indicating a radiation characteristic of the antenna 2 of the mobile station for a signal transmitted from the antenna 2 of the mobile station. R2 denotes a space distribution indicating a radiation characteristic of the antenna 2 of the mobile station for a signal received in the antenna 2 of the mobile station.
Next, an operation is described.
As shown in FIG. 7, in a signal transmission operation in which a desired signal is transmitted from the mobile station to the base station, a transmission directivity inherently held in the antenna 2 of the mobile station is used in the mobile station as it is. That is, a space distribution T1 indicating a radiation characteristic of the antenna 2 is determined according to the transmission directivity. Therefore, a signal is transmitted from the antenna 2 of the mobile station according to the space distribution T1. In this case, because the space distribution T1 is uniform in all directions, the signal is equally transmitted in all directions.
In contrast, as shown in FIG. 7, a directivity control of an adaptive antenna is performed for the antenna 1 of the base station to form a space distribution R1, which indicates a radiation characteristic of the antenna 1 for a signal received in the antenna 1 of the base station, as a reception directivity. In this space distribution R1 denoting the reception directivity of the antenna 1, a main beam of the antenna 1 is directed in a desired signal coming direction to receive a desired signal directly coming from the mobile station, and the reception directivity toward the interference wave source 3 is set to zero in cases where a reflected wave is transmitted from the interference wave source 3. Therefore, even though a reflected wave output from the interference wave source 3 comes as an interference wave to the antenna 1 of the base station, the reception of the interference wave in the antenna 1 can be prevented according to the space distribution R1 of the radiation characteristic of the antenna 1, and a desired signal directly coming from the mobile station is received in the antenna 1 at a prescribed communication quality.
Also, as shown in FIG. 8, in a signal transmission operation in which a desired signal is transmitted from the base station to the mobile station, a directivity control of an adaptive antenna is performed for the antenna 1 of the base station to form a space distribution T2, which indicates a radiation characteristic of the antenna 1 for a signal transmitted from the antenna 1 of the base station, as a transmission directivity. The space distribution T2 denoting the transmission directivity of the antenna 1 is determined by multiplying the space distribution R1, which indicates the reception directivity of the antenna 1, by an assumed weighting factor. Therefore, in this space distribution T2 of the radiation characteristic of the antenna 1, a main beam of the antenna 1 is directed in a desired signal transmitting direction to directly transmit a desired signal to the mobile station, and the transmission directivity toward the interference wave source 3 is set to zero. Therefore, no signal is output from the antenna 1 of the base station to the interference wave source 3 according to the space distribution T2 of the radiation characteristic of the antenna 1, and no reflected wave is output from the interference wave source 3.
In contrast, as shown in FIG. 8, in the antenna 2 of the mobile station, a reception directivity inherently held in the antenna 2 of the mobile station is used in the mobile station as it is, and a space distribution R2 of a radiation characteristic of the antenna 2 is determined according to the reception directivity. Therefore, a desired signal directly transmitted from the antenna 1 of the base station is received in the antenna 2 of the mobile station according to the space distribution R2. In this case, because the space distribution R2 is uniform in all directions, signals arriving from all directions are equally received in the antenna 2 of the mobile station.
As is described above, in the conventional mobile communication system shown in FIG. 7 and FIG. 8, the directivity control of the adaptive antenna is performed for the antenna 1 of the base station in the signal transmission and reception operations. Therefore, the reception of the interference wave, which is output from the interference wave source 3, in the antenna 1 of the base station and the antenna 2 of the mobile station can be prevented, and a communication quality in the signal transmission and reception operations can be improved.
Next, a case that a problem occurs in the conventional mobile communication system is described.
FIG. 9A is another explanatory view showing a signal transmission from a mobile station to a base station in the conventional mobile communication system shown in FIG. 7 and FIG. 8, and FIG. 9B is another explanatory view showing the reception of a signal output from a base station in a mobile station in the conventional mobile communication system shown in FIG. 7 and FIG. 8. In FIG. 9A and FIG. 9B, 4 indicates an interference wave source denoted by an antenna of a second mobile station such as a portable telephone or a portable information terminal.
As shown in FIG. 9A, in the signal transmission from the mobile station to the base station, the directivity control of the adaptive antenna is performed for the antenna 1 of the base station to form the space distribution R1, which indicates the radiation characteristic of the antenna 1 for a signal received in the antenna 1, as the reception directivity. That is, in this space distribution R1, a main beam of the antenna 1 is directed in a desired signal coming direction, and the reception directivity toward the interference wave source 4 is set to zero in cases where a reflected wave is transmitted from the interference wave source 4. Therefore, the reception of an interference wave, which comes from the interference wave source 4, in the antenna 1 of the base station can be prevented.
However, as shown in FIG. 9B, in the signal transmission from the base station to the mobile station, the directivity control of the adaptive antenna is performed for the antenna 1 of the base station to form the space distribution T2, which indicates the radiation characteristic of the antenna 1 for a signal transmitted from the antenna 1, as the transmission directivity. That is, the space distribution T2 is determined by multiplying the space distribution R1, which indicates the reception directivity of the antenna 1, by an assumed weighting factor. Therefore, in this space distribution T2 indicating the transmission directivity of the antenna 1, a main beam of the antenna 1 is directed in a desired signal transmitting direction, and the transmission directivity toward the interference wave source 4 is set to zero. In this case, though no signal is transmitted from the antenna 1 of the base station to the interference wave source 4, because a signal is output from the interference wave source 4 as an interference wave, the interference wave output from the interference wave source 4 is received in the antenna 2 of the mobile station. Therefore, a communication quality in the conventional mobile communication system cannot be maintained.
As is described above, because the conventional mobile communication system has the above configuration, in cases where an antenna of a second mobile station functions as the interference wave source 4, the interference wave output from the interference wave source 4 is received in the antenna 2 of the mobile station in the signal transmission from the base station to the mobile station. Therefore, there is a problem that a communication quality in the conventional mobile communication system cannot be maintained.
Also, because the directivity control of the adaptive antenna is performed for the antenna 1 of the base station in both the signal reception operation and the signal transmission operation performed in the base station, there is another problem that a calculation processing of the directivity control performed in the base station is complicated.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional mobile communication system, a mobile communication system in which a communication quality in the communication between a base station and a mobile station is maintained even though another mobile station functions as an interference wave source and in which an amount of calculation processing in the base station is reduced. Also, the object of the present invention is to provide a communication method performed in the mobile communication system.
The object is achieved by the provision of a mobile communication system comprising an antenna of a base station for performing a directivity control of an adaptive antenna in a signal reception of the base station to obtain a radiation characteristic of the antenna in which a main beam is directed in a desired signal coming direction and in which a directivity in a direction of an interference wave source is set to zero, and an antenna of a mobile station for performing the directivity control of the adaptive antenna in a signal reception of the mobile station to obtain a radiation characteristic of the antenna of the mobile station in which a main beam is directed in a desired signal coming direction and in which a directivity in a direction of the interference wave source is set to zero.
In the above configuration, in cases where a second mobile station functions as the interference wave source, an interference wave is transmitted from the interference wave source to the antenna of the mobile station in the signal reception of the mobile station. However, because the directivity of the antenna of the mobile station in the direction of the interference wave source is set to zero, the interference wave is not received in the antenna of the mobile station.
Accordingly, even though a second mobile station functions as the interference wave source, the reception of the interference wave output from the interference wave source can be prevented in both the antenna of the base station and the antenna of the mobile station, and a high communication quality in both the base station and the mobile station can be maintained.
Also, because no directive control is performed for the antenna of the base station in a signal transmission of the base station, an amount of calculation processing required in the base station can be reduced, a processor arranged in the base station can be simplified, and an electric power consumed in the base station can be reduced.
The object is achieved by the provision of a communication method of a mobile communication system comprising the steps of performing a directivity control of an adaptive antenna for an antenna of a base station in a signal reception of the base station to obtain a radiation characteristic of the antenna in which a main beam is directed in a desired signal coming direction and in which a directivity in a direction of an interference wave source is set to zero, and performing the directivity control of the adaptive antenna for an antenna of a mobile station in a signal reception of the mobile station to obtain a radiation characteristic of the antenna of the mobile station in which a main beam is directed in a desired signal coming direction and in which a directivity in a direction of the interference wave source is set to zero.
Accordingly, even though a second mobile station functions as the interference wave source, the reception of an interference wave output from the interference wave source can be prevented in both the antenna of the base station and the antenna of the mobile station, and a high communication quality in both the base station and the mobile station can be maintained.
Also, because no directive control is performed for the antenna of the base station in a signal transmission of the base station, an amount of calculation processing required in the base station can be reduced, a processor arranged in the base station can be simplified, and an electric power consumed in the base station can be reduced.
It is preferred that the communication method further comprises the step of performing a directivity synthesizing control for the antenna of the base station in a signal transmission of the base station to obtain another radiation characteristic of the antenna of the base station in which the main beam is directed in the same desired signal coming direction as that in the signal reception of the base station.
Accordingly, a gain for a signal output from the antenna of the base station according to the directivity of the antenna of the base station is increased, and a desired signal can be transmitted from the base station to the mobile station even though the mobile station is placed far away from the base station. Also, an area of a plurality of mobile stations, with which one base station communicates, can be widened.
It is preferred that the communication method further comprises the step of performing a diversity control for the antenna of the mobile station in a signal transmission of the mobile station to select one adaptive antenna, in which an electric power of a received signal is high, from a plurality of adaptive antennas composing the antenna of the mobile station and to transmit a signal from the selected adaptive antenna.
Accordingly, an electric power of a signal transmitted from the selected adaptive antenna to the base station can be increased, and an area of a plurality of mobile stations, with which one base station communicates, can be widened.